1. Field of the Invention
The present invention relates to the field of hydrodynamic bearing spindle motors for disc drive data storage devices and, more particularly, to a motor having a lubricant exhibiting reduced bubble forming tendency.
2. Description of the Related Art
Disc drive data storage devices, known as xe2x80x9cWinchesterxe2x80x9d type disc drives, are well known in the industry. In a Winchester disc drive, digital data is written to and read from a thin layer of magnetizable material on the surface of rotating discs. Write and read operations are performed through a transducer that is carried in a slider body. The slider and transducer are sometimes collectively referred to as a head, and typically a single head is associated with each disc surface. The heads are selectively moved under the control of electronic circuitry to any one of a plurality of circular, concentric data tracks on the disc surface by an actuator device. Each slider body includes a self-acting air bearing surface. As the disc rotates, the disc drags air beneath the air bearing surface, which develops a lifting force that causes the slider to lift and fly several microinches above the disc surface.
In the current generation of disc drive products, the most commonly used type of actuator is a rotary moving coil actuator. The discs themselves are typically mounted in a xe2x80x9cstackxe2x80x9d on the hub structure of a brushless DC spindle motor. The rotational speed of the spindle motor is precisely controlled by motor drive circuitry, which controls both the timing and the power of commutation signals directed to the stator windings of the motor. Typical spindle motor speeds have been in the range of 3600 RPM. Current technology has increased spindle motor speeds to 7200 RPM, 10,000 RPM and above.
One of the principal sources of noise in disc drive data storage devices is the spindle motor. Disc drive manufacturers have recently begun looking at replacing conventional ball or roller bearings in spindle motors with xe2x80x9chydroxe2x80x9d bearings, such as hydrodynamic or hydrostatic bearings. A hydro bearing relies on a fluid film which separates the bearing surfaces and is therefore much quieter and in general has lower vibrations than conventional ball bearings. A hydrodynamic bearing is a self-pumping bearing that generates a pressure internally to maintain the fluid film separation. A hydrostatic bearing requires an external pressurized fluid source to maintain the fluid separation. Relative motion between the bearing surfaces in a hydro bearing causes a shear element that occurs entirely within the fluid film such that no contact between the bearing surfaces occurs.
In a hydro bearing, a lubricant, such as a liquid or a gas, provides a bearing surface between a stationary member of the housing and a rotating member of the disc hub. Typical lubricants include oil or ferromagnetic fluids. Hydro bearings spread the bearing surface over a larger surface area in comparison with a ball bearing assembly, which comprises a series of point interfaces. This is desirable because the increased bearing surface decreases wobble or run-out between the rotating and fixed members.
In hydrodynamic fluid bearings, a principle design issue is preventing leakage of the lubricating fluid from the bearing during operation. Leakage can result from the continuous pressure cycling during start-stop processes of the spindle motor. At high pressures, air gets trapped in the lubricating fluid, and when the pressure is reduced, the trapped air forms bubbles and/or foam in the fluid. This bubble formation results in premature failure of the bearing and leakage of the lubricating fluid. In addition, foaming and/or bubble formation reduces the stiffness of the hydrodynamic bearing, increases the response time, reduces heat transfer, and results in a higher operating temperaturexe2x80x94all of which are undesired in hydrodynamic bearing operation.
Therefore, there exits a need in the art for a hydrodynamic fluid bearing having lubricants exhibiting a reduced bubble-forming tendency.
The disc drive data storage system of the present invention includes a housing having a central axis, a stationary member that is fixed with respect to the housing and coaxial with the central axis, and a rotatable member that is rotatable about the central axis with respect to the stationary member. A stator is fixed with respect to the housing. A rotor is supported by the rotatable member and is magnetically coupled to the stator. At least one data storage disc is attached to and is coaxial with the rotatable member. A hydro bearing interconnects the stationary member and the rotatable member and includes a lubricating fluid comprising a base fluid and an additive for reducing the tendency of bubble formation in the lubricating fluid due to sudden pressure fluctuations.